1. Field of the Invention
The present invention relates generally to a system and method for synchronization of multiple oscillators, and relates more particularly to synchronization independent oscillators with periodic waveforms to obtain a multiphase oscillator.
2. Description of Related Art
A number of applications call for synchronization of phases in a multiphase system. Various types of power and communication systems, for example, include oscillators oscillating at a particular frequency or providing signals over a particular time interval to achieve the goals of the given system.
One example of a system in which various phases are synchronized to achieve better system performance is an interleaved multiphase switching power supply. When the various phases of the switching power supply are properly synchronized, advantages such as reduced input current ripple, reduced peak output current and higher frequency output ripple current can be obtained. The higher frequency output ripple current permits easier filtering of the output current to remove the ripple component to produce a well-regulated DC output.
It can be particularly challenging to synchronize multiple phases of such a switching power converter when a switching power converter is a variable frequency switching power supply. The various phases of the variable frequency switching power supply act as free running oscillators, the frequency of which is controlled by the amplitude of voltages or currents switched in the various phases. As load demand changes, switching frequency changes for each of the phases and maintaining synchronization of the phases in all situations can be difficult.
In addition, the synchronization of phases to obtain a desired phase angle separation depends upon the number of phases that are paralleled together. For example, if a multiphase system, such as the above-described switching power supply, has two phases, the phase angle difference between the waveforms in each phase should be 180°. For a three-phase system, the phase angle difference between the waveforms in each phase should be 120°. In general, the phase angle separation between the waveforms in each of the phases is equal to 360°/N, where N is the number of phases in the multiphase system.
One way to correct for mismatch in phase separation is to employ a Phase Locked Loop (PLL) to maintain an appropriate phase angle separation. Such a concept is illustrated in U.S. Pat. No. 5,793,191, where a slave stage of a power converter is maintained 180° out of phase with a master power converter stage. This arrangement calls for special purpose components that can add to power converter cost, complexity and size. One drawback to this approach is the challenge of acquiring and maintaining a phase lock over a wide range of conditions. The PLL capture range must encompass the difference between the free-running frequencies of the master and slave(s), which may prove difficult or costly in practice. For example, if a change in load demand causes switching frequency to change rapidly, a large momentary frequency error may result, which can cause loss of phase lock.